Current regulating system



R. W. GILBERT CURRENT REGULATING SYSTEM Filed April 25. 1952 April 23,1957 2,790,132

ROSWELL iv. GILBERT INVENTOR.

a, x 4 AT RIVEYS United States Patent-O CURRENT REGULATING SYSTEMRoswell W. Gilbert, Montclair, N. J., assignor to Weston ElectricalInstrument Corporation, Newark, N. J., a corporation of New JerseyApplication April 25, 1952, Serial No. 284,387

1 Claim. (Cl. 323-4) This invention relates to regulated current sourcesand more particularly to a DC. current regulator circuit including anelectron tube and means compensating for the tube voltage ratio andcathode thermal influence thereon.

Numerous devices, particularly in the field of instru mentation, requirea steady, or standardized, source of direct current. Regulated D.-C.supplies are available for this purpose but present devices of thisclass generally include regulation against load current burden changesas well as against line voltage changes. The present invention isdirected to the provision of a novel and simplified circuit arrangementproviding a regulated DC. current supply for application in thosedevices which involve substantially constant burden resistances. Mynovel circuits are derived by applying compensation to a basic cathodefollower circuit and may be termed compensated cathode followercircuits. They include a degenerative bridge feedback network providingcompensation for the voltage ratio (,u.) of a tube and compensation ofthe cathode thermal influence upon such tube voltage ratio.

An object of this invention is the provision of a simplified D.-C.current regulator circuit.

An object of this invention is the provision of a regulated D.-C.current supply including an electron tube and a bridge type degenerativefeedback circuit compensating for the tube voltage ratio and cathodethermal influence thereon.

An object of this invention is the provision of a regulated D.-C.current source of the type comprising an electron tube and a cathodefollower circuit and including an R-C feedback circuit coupling theplate to the grid through a grid series resistor, said R-C circuithaving a time-constant matched to the thermal time-constant of thecathode.

An object of this invention is the provision of a regulated D.-C.current source comprising an electron tube, voltage sources energizingthe tube electrodes, circuit, elements connected to the tube electrodesto form a cathode follower circuit, a degenerative feedback circuitconnected between the tube plate and cathode and having a voltageattenuation ratio equal to the reciprocal of the tube voltage ratio, andan R-C feedback path coupling the tube plate to the grid through a gridseries resistor, said R-C path having a time-constant matched to thethermal time-constant of the tube cahode.

These and other objects and advantages will become apparent from thefollowing description when taken with the accompanying drawings. It willbe understood thev drawings are for purposes of illustration and are notto be construed as defining the scope or limits of the invention,reference being had for the latter purpose to the appended claim.

In the drawings wherein like reference characters de note like parts inthe several views:

Figure 1 illustrates an elementary circuit of the oath ode follower typeconnected to a steady reference poin Figure 2 is similar to Figure 1 towhich has been added a plate-cathode L network proportioned to supplythe operating differential arising by reason of the change in thecathode potential as the plate voltage changes;

Figure 3 is similar to Figure 2 and including a conventional pentodeconnection to correct output current variations arising by reason ofvoltage inputs that are high or low with respect to the input levelwhere regulation is perfect;

Figure 4 is similar to Figure 2 but including an R-C feedback pathcoupling the plate to the grid through a grid series resistor tocompensate for the thermal drift of the tube cathode;

Figure 5 is a circuit diagram illustrating my compensated cathodefollower circuit including a gas voltage regulator tube constituting thereference potential source;

Figure 6 is an equivalent four-terminal network; and

Figure 7 is a circuit diagram of a regulated D.-C. current supply madein accordance with this invention.

Electronic regulators generally may be expressed as four terminalnetworks having a finite transfer immittance for direct current but zerotransfer immittance for alternating current, or differential, componentsof input. When the zero transfer function is valid down to zerofrequency, the output of the network will not be influenced by changesin input resulting, therefore, in a steady D.-C. output level regardlessof input variations, within functional limits of the network. It will beappreciated that such a network must include at least one element thathas a difierential resistance unequal to its volt-ampere ratio. Incommon use for this purpose are such devices as lamp filaments,batteries, rectifier elements, thermistors and saturated thermioniccathodes. However, the most convenient such device is the cold-cathodegas tube which, in effect, presents a relatively constant potential inseries with a nominal amount of differential resistance.

Reference is now made to Figure 1 which is a. circuit diagram of anelementary current regulator, of the cathode follower class, connectedto a steady reference potential such as the battery 10. The grid 11, ofthe tube 12, is connected to the battery and the output load device,here shown as the resistor 14, is connected in series with theindicating instrument 13 between the other side of the battery and thetube cathode 15. The input circuit to the regulator comprises theterminals 16, 17, adapted for connection to a source of plate voltage inaccordance with the indicated polarity. As the plate voltage changes toalter the cathode current, the resulting change in cathode potentialwith respect to the grid corrects the plate resistance to minimize thecurrent change, except for a small operating differential. Perfectregulation requires a vanishingly small operating differential whichwould obtain only if the tube transconductance be infinite.

Figure 2 illustrates a modification of the Figure 1 circuit.Specifically, a plate-cathode L network, comprising the resistors 18,19, has been added to supply the operating differential above mentioned.As the plate voltage changes the added network transmits the change,properly attenuated, to the cathode circuit to exactly supply thegrid-cathode potential shift required to correct the change in terms ofcathode current. Correction obtains when the voltage-attenuation ratioof the added network equals the reciprocal of the tube voltage ratio(,u.) The feedback direction is degenerative and cannot result ininstability. Compensation is susceptive to variations in the tubevoltage ratio with plate voltage level, the tube voltage ratio generallyincreasing with increasing plate voltage. Thus, characteristically, thesystem will be over-corrected at input levels higher than design centerand under-corrected for input levels lower than design center. Whenproperly adjusted at the design center input level, the output currentwill have a second order variation causing a slight decrease with actualinputs that are high or low relative to the input level Where regulationis perfect. This second order variation is very small and variesapproximately as the square of the input level deviation from designcenter. it can easily be made smaller than 25x10 times the input levelchange-ratio squared. Theoretically, this error can be reduced byrecourse to the pentode connection shown in Figure 3. A study of Figure3 will show that it is similar to Figure 2 with the addition of thepentode screen grid 20 to the electron tube. However, the Figure 3combination is rather superfluous in view of thermal. drift of thecathode as an intervening limitation. Further, the screen current demandof the pentode connection involves practical complications.

All circuit elements are subject to thermal drifting but thatoriginating by reason of the temperature coefiicient of the tube cathodecontact potential is the most obviously large. If the circuit isadjusted for optimum regulation against rapid input level changes thefollowing cathode temperature change with heater voltage will causedrift. Alternatively, it the circuit is adjusted to accommodate cathodetemperature drift as a steady state correction,

it will be subject to over-correction for rapid input level changes.This effect could be minimized by separate regulation of the heatersupply voltage but as a sacrifice of circuit simplicity. Also, thestated effect could be reduced to a second order by a second tubeelement in compensating relationship but with similar objection.

As measured in typical types of electron tubes, the cathode contactpotential changes about 10 millivolts per percent of cathode heatervoltage change near design center. The thermal time constant i about 5seconds for small receiving tube cathodes. in a circuit using a 60 voltgas tube as a reference potential, this would produce a thermal drift ofM the input change, or an output change of 0.25 percent for a typicalinput level change of '15 percent. This is limiting to a systemotherwise capable of regulating the same input level change to betterthan 0.05 percent and, therefore, requires correction.

My simple arrangement for compensating the cathode thermal drift isshown in Figure 4- which basically is similar to Figure 2. The cathodethermal drift compensator comprises the resistor 22 and capacitor 23forming an RC feedback path coupling the plate 21 to the grid 11 througha grid series resistor 24-. The RC circuit has a tir'neconstant matchedto the thermal time constant of the tube cathode. This feedback circuit,degencrates the tube voltage ratio slightly, requiring some additionalcompensation for instantaneous input level changes, but it becomesineffective in the study state condition allowing the slight measure ofover-compensation to include correction for cathode thermal drift. Byproper adjustment of this feedback magnitude, and the timeconstant andmagnitude of the main compensation feedback, the circuit is made stablefor both rapid and slow changes of input level.

Thus far, for simplicity of illustration, the reference potentialsources have been shown as batteries. In practice, however, gasvoltage-regulator tubes are employed for the purpose. The differentialresistance of such gas tribes is slightly influential upon regulationbut such effects can be included in the compensation adjustment.

Figure 5 illustrates a circuit diagram of my compensated cathodefollower arrangement including a gas voltage-regulator tube 25 and withthe compensating bridge elements here identified "2 R1, R2, R3 and R0,the latter being the differential resistance of the regulator tube 25.

Figure 6 illustrates the equivalent, four-terminal network with the tubevoltage ratio t) in the feedback loop with the voltage transfer function(13) of the bridge.

The transfer conductance (G) of the system maythcn be expressed as G RDwhere E is the input voltage,

I is the output current, and

Rp is the plate resistance of the tube 12.

and the system becomes independent of Rp.

The voltage transfer function (5) of the bridge is:

so that the bridge relationship should be:

RiR3R0R2 It is pointed out that if the voltage reference tube .25 wereideal and Ro=0, Equation 4 would reduce to:

It will now be apparent that the circuit of Figure 5, including thepractical voltage reference tube 25, maybe combined with the cathodethermal compensating elements 22, 23 and 24 of Figure 4 in a practicaloperating circuit as shown in Figure 7. In this circuit a dual-triodctube, Type 6SN7, is used; one triode section serves as a current controltube as in Figure 5, and the other triode section is diode-connected toserve as a supply-power rectifier in a conventional manner. The voltagereference tube is a Type 5651 which is a very stable gaseous-dischargevoltage regulator. The output load circuit includes a resistor Rs ofappropriate value, the voltage drop across which can be balanced againsta standard cell 31 by means of a galvanometer 32 through a switch 33 tostandardize.

the output current. The primary adjustment of output current is thevariable resistor 39. A second variable potentiometer resistor 40 servesto balance the regulation of the system to the point where changes ofoutput current in response to line voltage changes are a minimum. Thecircuit values indicated are designed for an output current of 5milliamperes.

Having now described my invention in detail in accordance with thepatent statutes what I desire to protect by Letters Patent of the UnitedStates is set forth in the following claim.

I claim:

An arrangement for maintaining a substantially constant flow of currentin a circuit energized by a D.-C. voltage source that is subject tovariations, said arrange.- ment comprising an electron tube having ananode connectcd to the positive side of said D.-C. voltage source, acathode and a control electrode; a reference source of constant D.-C.voltage having one side connected to the said control electrode and theother side connected directly to the negative side of the said D.-C.voltage source; a first resistive connection between the cathode and thenegative side of said D.-C. voltage sourcc and a second resistiveconnection between the said anode and cathode, the values of the saidresistive connectionsv being such as to maintain a substantiallyconstant current flow in the cathode circuit throughout the range ofvariations of said D.-C. voltage source; and an R-C circuit coupling thesaid anode to the control electrode, said R-C circuit 5 6 having a dne-constant matched to the thermal time con- FOREIGN PATENTS cathode472,326 Great Britain Sept. 22, 1937 References Cited in the file ofthis patent OTHER REFERENCES UNITED STATES PATENTS 5 Publicationsentitled The Cathode Follower as 8. Volt- 2,281,205 Shock Apr. 28, 1942age Regulator, by A. P. Willmore, Electronic Engineer- 2,583,837Hadfield Jan. 29, 1952 ing of September 1950, pp. 399-400.

